MEMS devices designed to perform optical functions have been developed using a variety of approaches. According to one approach, a deformable deflective membrane is positioned over an electrode and is electrostatically attracted to the electrode. Other approaches use flaps or beams of silicon or aluminum which form a top conducting layer. With optical applications, the conducting layer is reflective while the deflective membrane is deformed using electrostatic force to direct light which is incident upon the conducting layer.
More specifically, a MEMS technology termed Diffractive Light Devices (DLDs) produce colors based on the precise spacing of a pixel plate to related lower (and possibly upper) plates. This spacing is the result of a balance of two forces: electro-static attraction based on voltage and charge on the plates, and a spring constant of one or more “flexures” maintaining the position of the pixel plate away from the electrostatically charged plate. One traditional approach for controlling the gap distance is to apply a continuous control voltage to the electrodes, wherein the control voltage is increased to decrease the gap distance, and vice-versa. However, precise gap distance control may be affected by a variation in operating temperatures experienced by the DLD.
This variation is often caused by light absorption. As the DLD operates, light of the desired wavelength is emitted from the DLD. Much of the rest of the light that enters the DLD is absorbed. This light absorption causes the temperature of the DLD to rise.
As the temperature of the DLD rises, the spring constant of the flexures decreases. As a result, at an elevated temperature the spring force opposing a given electrostatic force is smaller. Consequently, the application of a given electrostatic force results in a smaller resulting gap at elevated temperatures. As the resulting gap distance changes with temperature so does the output of the DLD. This variation in the gap distance reduces the precision of the DLD and consequently causes the resultant color to shift undesirably.
Several approaches have been undertaken to compensate for this temperature change. Many of these approaches attempt to regulate the input voltage by monitoring the output of the DLD and modulating the control voltage to obtain a temperature compensated voltage gap. These approaches are often complicated and involve expensive monitoring and processing circuitry.